As known in the prior art, coated rolls are used in paper machines and in paper finishing devices in highly different applications. As examples of such applications, soft rolls of calenders, for example of supercalenders, should be mentioned. Usually the soft coatings on rolls are made of organic polymers or mixtures thereof, which often also include inorganic elements. The soft coatings on rolls are often made of a composite structure, which comprises layers made of different materials.
Supercalenders consist of a number of rolls arranged one above the other (adjacent ones of which are in nip-defining relationship), and the rolls are alternatingly soft and hard. In this manner, the paper web runs successively through a number of nips. In a typical supercalender, the hard rolls are made of metal, usually steel and/or cast iron, and the soft rolls are paper-filled or fabric-filled. Since the metal rolls in the pairs of rolls are usually heated in order to obtain good calendering results, one problem in the calendering has been the poor ability of the resilient paper-filled or fabric-filled rolls to endure high temperatures. Owing to this, polymer-coated rolls have been introduced in calendering.
The frame of polymer-coated rolls does not have a uniform rigidity in the longitudinal direction of the roll, but the end areas are more rigid than the middle area of the roll frame. FIG. 1 is a schematic illustration of the state of deformation in a radial direction (i.e., cross-sectional deformation) of the roll frame of a polymer roll during deformation in the middle area and at one end of the roll, which arises due to the difference in rigidity. In this manner, in a polymer roll, a higher load arises in the lateral areas than in the middle when such rolls are used in situations in which the paper web or equivalent fibrous material layer is run through a nip between such a polymer-coated roll and a hard roll, and in particular when there is a roll at each side of a polymer-coated roll, for example in supercalendering. In the case of supercalendering, the quality in the lateral areas of the web can suffer as a result of the higher load effective in the lateral areas of the paper web. In calenders, attempts have been made to solve this problem by controlling the nip load, but by means of the control of the nip load alone, it has not been possible to rectify this problem to a sufficient extent.
With respect to the prior art, Slotten (U.S. Pat. No. 5,662,574) describes a pressure roller including an elongate central core, a roller body arranged on the central core and a thin outer shell arranged on the roller body. The central core of the roller of Slotten is not deformable such that it deforms less in lateral areas than in a middle area during loading of the roll. Rather, the central core is made "of a material and of a construction that resists deformation" (see col. 2, lines 33-34) and as such, does not operatively deform differently in the lateral areas and in a middle area.
Pessen (U.S. Pat. No. 3,750,246) describes a composite roll A for controlled deflection in calendering operations to correct distortion of the roll during calendering. Composite roll A includes a metal shaft or core member B and a metal outer cylindrical shell. The core member B is not deformable such that it deforms less in lateral areas than in a middle area during loading of the roll. Rather, the core member B is "substantially rigid" (see claim 1, lines 1-2) and as such, would not operatively deform differently in the lateral areas and in a middle area.
Rise (U.S. Pat. No. 5,092,235) describes rollers 40,42 defining a high pressure nip through which ink-containing sheet material is passed. Each roller 40,42 may include a core 54, an elastomer 56 arranged on the core 54 and a shell 50 arranged on the elastomer 56. Core 54 of the rollers 40,42 of Rise is not deformable such that it deforms less in lateral areas than in a middle area during loading of the roll. Rather, the core 54 is made of "a rigid, preferably non-compliant, material such as steel" (col. 6, lines 22-23) and as such, does not operatively deform differently in the lateral areas and in a middle area.
Lehtonen (U.S. Pat. No. 5,412,870) shows a coated roll having a polymer coating which may be variable along the axial length of the roll. The roll is not deformable such that it deforms less in lateral areas than in a middle area during loading of the roll. Rather, the roll provides a uniformly rigid substrate in the axial length of the roll such that the hardness of the roll is determined solely by the manner in which the coating is applied to the outer surface of the roll, i.e., the surface hardness of the roll being regulated sole by using the functions derived according to Lehtonen (col. 5, lines 24-31).
Great Britain Patent No. 795,523 describes a pressure rollers including a core a', a rubber layer b' having a uniform thickness and a resilient cover c' having a variable thickness. Core a' of the rollers is not deformable such that it deforms less in lateral areas than in a middle area during loading of the roll. Rather, the core a' is made of "hard steel" (page 2, lines 97-100) or another "non-resilient" material (page 3, lines 11-15), and as such, does not operatively deform differently in the lateral areas and in a middle area. Moreover, the roll has a solid roll frame which is generally, uniformly rigid in the axial direction.
IBM Bulletin, Vol. 27, No. 1A, June 1984, Uniform Pressure Roll, shows a roll with a solid roll core or frame. The roll does not have a tubular roll frame having a non-uniform rigidity.